Optical, dielectric and thermal properties of nanoscaled films of polyalkylsilsesquioxane composites with star-shaped poly(ε-caprolactone) and their derived nanoporous analogues

Nanoscaled films of poly(methylsilsesquioxane-co-ethylenylsilsesquioxane) (PMSSQ-BTMSE) and polymethylsilsesquioxane (PMSSQ) were prepared from the respective soluble prepolymers, and their thermal, optical, and dielectric properties were characterized. The PMSSQ-BTMSE nanofilm containing an ethylenyl bridge comonomer unit BTMSE was found to be thermally more stable than the nanofilm of PMSSQ, a representative polyalkylsilsesquioxane. Further, in comparison to the PMSSQ film the PMSSQ-BTMSE film exhibits a larger refractive index and a larger dielectric constant but a smaller out-of-plane thermal expansion coefficient, when both prepolymers are cured under the same conditions. These characteristics of PMSSQ-BTMSE films are due to the ethylenyl bridge provided by the BTMSE comonomer unit, which promotes the formation of a tighter, more perfect network structure in cured PMSSQ-BTMSE films. Composite films were prepared by solution-blending the pore generator (referred to as the porogen), a star-shaped poly(ε-caprolactone), with the soluble prepolymers then drying the resulting solution, and the nature of their curing reactions and extent of porogen calcination were investigated. It was found that the thermal curing and calcination processes of the composite films successfully produce PMSSQ-BTMSE and PMSSQ films containing pores that are about 400 nm in diameter. It was confirmed that the presence of these generated nanopores significantly reduces the refractive indices and the dielectric constants of the dielectric films but increases their out-of-plane thermal expansivity, depending on the initial porogen loading. The porosities of the nanoporous dielectric films were estimated from the measured refractive indices. The surface topographies of these films were also investigated, giving information about the sizes of the pores generated in the films. It was also found that prior to calcination the presence of the porogen increases the refractive index and dielectric constant of the composite dielectric films because of its inherent high polarizability, and also increases the thermal expansivity of MSSQ-BTMSE composite films but very slightly decreases the thermal expansivity of MSSQ composite films. It is demonstrated that PMSSQ-BTMSE films and the related nanometer scale nanoporous films are candidates for use as low and ultra-low dielectric interlayers in the fabrication of advanced microelectronic devices.